During the last decade oriented strand board (OSB) has become a particularly important wood product in the home construction industry. Since its appearance in 1978, OSB has become the most rapidly growing wood-based composite product. OSB is primarily used as a structural panel, which in the past was dominated by softwood plywood. In 1999, OSB production passed softwood plywood in North America for the first time. Approximately 20.325 billion SF of OSB were produced in 1999 compared 20.275 billion SF of softwood plywood.
Presently, adhesives used in binding systems for OSB and other composite wood products include phenol-formaldehyde (PF), urea-formaldehyde, polyvinyl acetate, resorcinol-formaldehyde, polymeric diphenylmethane diisocyanate (pMDI), and hot melt adhesives. Approximately 18% of OSB production in North America utilizes a polyisocyanate-based adhesive.
Polyisocyanate adhesive systems provide significant advantages over the more traditional PF-based adhesive systems. These advantages include a relatively fast cure, insensitivity to moisture, tolerances for seasonal changes, and final composite properties such as thickness swell, durability, internal bond, and modulus of the finished products. However, the polyisocyanate-based adhesives raise an issue of cost and issues related to production. Polyisocyanate adhesives are generally more expensive (for comparison, PF resin is about 0.35/lb, and most popular polyisocyanates are about 0.65/lb). Also, OSB boards produced with polyisocyanate have a tendency to adhere to the platens during manufacture. This adhesion can damage or even ruin the board product, and the subsequent cleanup of the contaminated platens is difficult, time-consuming and costly.
Commercial suppliers of polyisocyanate-based adhesives have long sought to produce an effective self-releasing polyisocyanate product for composite wood products including OSB. Various attempts have been made to overcome the platen adhesion problem without sacrificing other desirable board properties. The addition of release agents such as oils, wax polishes, silicone and polytetrafluoroethylene directly to the platen surface has helped somewhat. Another approach involves the addition of release agents to the adhesive system.
The use of polyisocyanate adhesives can also raise health and work-safety environmental issues. Polyisocyanates can react with moisture on the skin or with moisture in the lungs, if inhaled as atomized polyisocyanate or isocyanate-coated wood dust. One popular type of polyisocyanate used in the composite wood industry is polymeric diphenylmethane diisocyanate (pMDI). pMDI can contain a wide range of oligomers of varying amounts, typically 40–60% of monomeric 4,4′-diphenylmethane diisocyanate (MDI). As the curing reaction with water occurs an intermediate reaction product, 4,4′-diaminophenylmethane (MDA), is produced. Because MDA is a relatively toxic chemical its presence needs to be maintained at a minimum. Also, to avoid hazardous work-place conditions cure rates should be relatively rapid and the curing reaction should approach completion with a minimum of non-reacted product as well as undesirable intermediate side products, e.g., MDI and MDA.
U.S. Pat. No. 5,942,058 describes the use of a fast-setting resin such as p-MDI with a slow-setting, co-adhesive such as a conjugated, triglyceride drying oil for making composite panels such as OSB, plywood, MDF, hardboard, and similar panels. U.S. Pat. No. 4,742,112 describes the use of liquid esters or vegetable oil extenders such as linseed oil, corn oil, trioctyl trimellitate, soybean oil, ditridecyl adipate, or dibutyl phthalate to enhance compatibility with cable greases. The vegetable oil is added to lower the viscosity of the high viscosity hydrocarbon polymer polyol. Similarly, U.S. Pat. No. 6,352,661 describes the use of a high flash point diluent (>250° F.) such as aromatic hydrocarbon, phthalic acid derivative, parrafinic distillate solvent, propylene carbonate, chlorinated paraffin, or alkyl sulfonic ester of phenol to reduce the viscosity of polyisocyanate resins used to make wood composite products.
Given the relatively high cost of polyisocyanate adhesive, the platen sticking problem, and the health issues raised it is desirable to reduce the relative content of polyisocyanate in an adhesive composition. However, the reduction in polyisocyanate content must not compromise the performance of the finished products. These objectives remain quite a challenge to those in the industry.